Rivers seem to be something most cartographers struggle with, especially at first. So your challenge, should you choose to accept it, is to map a plausible - and interesting! - system of rivers. Topic, art style, and so on are yours to decide as long as it contains a river system. If you decide to use an unusual world (different geology, liquids, or laws of physics), spell them out - so the judges can decide whether or not your river system makes sense for your set of rules. This is probably best suited for a Lite challenge.